In a television transmission system, e.g., a satellite relay system, an FM, frequency modulation, system is often used. The reception RF, radio frequency, signal level in such a system is usually around the threshold level. Accordingly, if the reception signal level decreases slightly, that level becomes lower than the threshold level, and the picture and other aspects of a television signal is considerably deteriorated by spike noise peculiar to a television transmission, and it becomes difficult or impossible to reproduce a signal. Prior art receivers cannot usably receive signals from an obstructed dish such as an indoor dish, or produce usable reception from some of the weaker portions of a satellite footprint pattern.
In prior art practice, the individual modulation components of the signal are treated as if they were inseparable with respect to demodulation, and could not be separated during, but only after, demodulation. Thus a single demodulator, usually designated as a "video" demodulator, simultaneously demodulates video (picture), sync, color burst and audio subcarrier modulation components in prior-art receivers, with the audio subcarrier being further demodulated for its audio content by a subsequent demodulator, usually designated as an "audio" demodulator, operating from the output of the "video" demodulator, rather than directly from the RF or IF, intermediate frequency, signal. It will be shown that such individual modulation components of an FM signal can in fact be separated during demodulation and that such separation, contrary to the case of separation after demodulation, can lead to a lowering of overall system threshold.
In the prior art there are, as well as the conventional demodulation systems referred to above, references to systems for providing threshold extension for television signals by the use of a variable-bandwidth, i.e., adaptive, filter in conjunction with a conventional, e.g., limiter-discriminator, "video" demodulator, operating on all modulation components simultaneously, such that the bandwidth is large when the signal strength is at or above threshold, and narrow when the strength is below threshold. Like the conventional FM television receiving systems, such a system requires sufficient bandwidth, even in the narrowes mode, that no significant distortion results for any of the above-mentioned modulation components of the signal. Such a restraint on bandwidth narrowing makes it impossible to extend threshold sufficiently to reproduce many excessively-weak signals, and it is one object of the present invention to avoid such deleterious restraint on bandwidth, as will be indicated in more detail hereinafter.